In a large skillet, heat 1 tablespoon olive oil over medium-high heat. Add the onions, and cook them for 5 minutes or until they soften, stirring occasionally. Transfer the onions to a plate.

Place the skillet over high heat, and add the remaining 1 tablespoon olive oil. When the oil is very hot, add the grated summer squash and the garlic. Stir frequently, making sure the squash does not burn. After 2 minutes, take the skillet off the heat, and add the Parmesan cheese, salt and pepper.

Cut the tomatoes in half crosswise, and squeeze out the excess juice and most of the seeds. Chop the tomatoes, put them into a bowl, and season them with salt and pepper.

Preheat oven to 450F. On a floured surface, roll out each ball of dough to an 8-inch round. Put half of the tomatoes, then the squash, into the middle of one round. Sprinkle with half of the basil. Fill the second round of dough the same way. Moisten the edge of each round with water, and fold the calzone over, pressing with your fingers to seal the calzone tight. Place the calzones on a baking sheet sprinkled with cornmeal.

Bake the calzones for 15 minutes, or until they are golden brown. Serve hot. Serves 2.